Solar pool heaters survived the boom and bust solar years of the 1970s and 1980s. Today they are even popular and cost-effective in parts of the country where many people think solar is impractical.

In nearly every part of the country, solar pool heaters are a practical and economical way to make pools more comfortable and extend the outdoor swimming season. There are over 300,000 solar pool heaters in the United States, saving money and avoiding the pollution that gas and electric pool heaters cause. Still, this is a fairly small part of the total pool-heating market. Many of the approximately 6.6 million residential pools in the continental United States are heated. The cost of heating these and commercial and institutional pools runs to billions of dollars every year.

Solar pool heaters may cost more to install than conventional gas or electric pool heaters, but the higher purchase price is offset by low maintenance and no fuel costs. By circulating the water through the collectors at night, solar systems can even cool the pool during hot summers and thereby reduce evaporative water loss.

How Solar Pool Heaters Work
Many people prefer a pool temperature of around 80oF. This means pools need to be heated, even in Florida. Why throw money into chemicals, water, and cleaning expenses and not be able to use the pool? Many indoor pools also need to be heated.

Solar pool heaters are the simplest and least expensive solar water heating systems. The solar collectors raise the temperature of the water approximately 5oF-10oF between the inlet and the outlet of the collector. Because they do not have to achieve high temperatures, the collectors do not need to be glazed, or encased in a glass-covered, insulated box, like solar domestic hot water systems. In fact, on calm, hot days solar pool heaters operate so close to the ambient air temperature that they do not lose much heat to the environment. The collector transfers most of the solar energy that strikes it to the water, often operating at efficiencies of 80% and higher. A properly operating solar pool collector feels cool to the touch.

Solar-heated pool water heats up slowly as the water circulates continuously through the collectors. Over successive sunny days, the pool temperature will increase by 10oF-20oF. The system keeps heating the pool as long as the collector temperature is higher than the pool water temperature. During the middle of the summer it may be necessary either to bypass the collectors during the day to keep the pool from getting too hot, or to operate the system at night so that the collectors cool the pool.

Solar pool-heating systems (see Figure 1) work in conjunction with the pool pump, filter, and chlorinator (if used). The pump pulls water from the pool and forces it through the filter. The filtered water passes through a pressure relief valve and then a check valve. The check valve prevents backflow of water in the collectors from reentering the filter. From there the water flow is controlled by a bypass valve. When the valve is closed, water flows to the collectors and then continues its normal path back to the pool. When the valve is open, water bypasses the collector loop and flows to the conventional heater (if used) and chlorinator and then on to the pool. The bypass valve can be either a manual valve, or more conveniently, a valve operated by an automatic differential controller.

Figure 1. Schematic of a solar pool heating system

A controller monitors the water temperature at the check valve (or at any point between the pump and the bypass piping) and the temperature of the collector. As long as the pool temperature is lower than the collector temperature and the preset desired temperature, and the pump is operating, water will circulate in the collector loop. When the collector temperature drops (at night or during cold weather), the bypass valve opens. The collectors then automatically drain out the return.

There are also two manual isolation valves in the collector loop, one leading to the collectors and the other located on the return. These make it possible to isolate the collectors during servicing, while allowing the pool filter to continue to operate. A vacuum break or air ventat the highest point of the collectors allows air to enter or exit when the collectors fill and drain.

Types of Solar Pool Heater Collectors
As described above, most solar pool heater collectors are unglazed. If the pool is used all year in regions where freezing may occur and damage the equipment, then glazed collectors may be necessary. They are much more expensive, however. Glazed collector systems may use an antifreeze solution as the heating fluid instead of directly heating the pool water. The fluid passes through a heat exchanger, indirectly heating the pool water. These collectors are the same as those used in residential or commercial solar domestic hot water systems.

There are four types of unglazed solar pool heater collector (see Figure 2):

Metal tube-on-sheet panels, usually with copper tubes on a copper or aluminum sheet.

Figure 2. Types of solar collectors

Each system has its advantages and disadvantages. Ultimately, the choice of a system comes down to site-specific factors, such as the amount of space available for the collectors, and the individual user's preferences.

Metal tube-on-sheet collectors conduct more heat per ft2 of collector area, so they take up less space but are more susceptible to freezing. Copper collectors require careful monitoring of the pool pH level. If it falls below 7.2, becoming relatively acidic, the chlorine in the pool water may react with the copper, causing it to lose ionsto the water. High concentrations of these ions may cause a dark film to form on the pool walls. This film can only be removed by draining the pool and cleaning the walls, or even repainting them. Be sure to follow the operating and maintenance instructions for both the solar system and the pool itself. The solar collectors should be bypassed when adding pool chemicals and when chemically shocking the pool to kill bacteria. Wait until the pH returns to normal levels (between 7.4 and 7.8) before putting the collectors back on line.

Even though they are less efficient than metal collectors, plastic-panel and rubber-mat collectors are more commonly used. Plastic and rubber have thermal conductivities as much as 1,000 times lower than those of metals, so they require a larger collector surface area. However, since they are less expensive per ft2 than metal collectors, the overall capital cost is about the same, and they are not susceptible to corrosion from pH variations.

The life expectancy of collectors varies depending on the material they are made of. Plastic and rubber collectors should have UV inhibitors added to retard degradation from sunlight. They will typically last 10 to 15 years. Collectors that do not have any UV inhibitors may last only one year. Metal collectors will last decades if they are properly maintained.

Simple, low-cost, do-it-yourself systems can be made by simply laying out black plastic pipe in the sun. However, they may perform poorly compared to commercially available systems. Commercially available pool collectors must undergo stringent testing before they can be certified by the Solar Rating Certification Corporation (SRCC).

Collector and Pump Sizing
A number of factors determine the amount of collector area required. These include temperature, wind speed, relative humidity and insolation (incident solar radiation) regimes; pool size, location, and exposure to the sun and wind; how often and during which months the pool is used; whether a pool cover is used (highly recommended); the type of collector; how warm the users want the pool to be; and how much of the heating demand the system is designed to meet.

Putting a Cover on Heat Loss

Pools lose heat through convection (evaporation) and radiation from the pool surface, and by conduction from the sides or walls of the pool. How much heat is lost at any given time is determined by water temperature; ambient air temperature, relative humidity, and pressure; effective ambient radiant temperature (mostly sky temperature); wind flow across the surface; user activity (splashing causes greater evaporation); and ground temperature. Exposure to sunlight and cloud density also affects outdoor pool heat loss.

On average, outdoor pools lose almost 90% of their heat from the water surface: 70% by evaporation, and 20% by radiation to the sky. About 10% is lost by conduction from the sides and bottom. Heat loss from indoor pools is similar, but is somewhat controlled by the operation of the air heating and cooling, ventilation, and humidity control systems. Only around 3% of the heat input is lost by conduction in indoor pools.

Clearly, reducing heat loss from the pool surface is the primary consideration. A pool cover is the easiest and least expensive way to do this and also reduce water loss. Studies using DOE's Energy Smart Pools software (see Pools Get RSPEC from DOE) indicate that pool covers reduce water consumption by 30%-50%. For example, a cover could save as much as 4,000 gallons of water per year that would otherwise evaporate from a 450 ft2 pool.

Energy savings from installing a pool cover vary with how often the cover is on. As a rough rule of thumb, a cover used 50% of the time will save about 50% of the heating energy, a cover used 70% of the time will result in about a 70% savings, and so on. A pool cover makes sense even on an unheated pool. In some climates, pools can be heated to 80oF during the day simply through the diligent use of a cover.

We ran some pool cover calculations through the software for a 450 ft2 outdoor pool in Detroit, Michigan, kept at an average pool temperature of 80oF. Heating costs for a summer using propane came to about $1,390. Installing a pool cover and keeping it on all but 14 hours per week reduced the estimated summer heating cost to $130, a savings of $1,260. With natural gas and electric resistance heating the results were similar. Covering a natural gas-heated pool reduced estimated costs from $300 to $30 per summer, and covering an electrically heated pool reduced costs from $2,700 to $250 per summer.

Pool covers come in several different types. The simplest are placed over the pool by hand. Units with a dispensing roll, with or without a manual crank or electric motor, make covers convenient for small pools and practical for larger ones. There are bubble types, which resemble bubble packing, and vinyl covers with and without insulation.

Transparent and translucent (solar) covers work better than opaque covers because they allow sunshine to warm the pool through the day. A solar cover will raise pool temperatures between 5oF and 10oF (3oC-6oC), depending on the type of cover, when and how long it is used during the day, and how much the sun shines on the pool. Pool covers that contain ultraviolet (UV) inhibitors will last approximately 3 to 5 years. Covers that don't may last only one season. Pool covers should be used whenever the pool is not in use and should be removed completely, for safety reasons, before anyone enters the pool.

Other Ideas

Wind flow over the pool surface significantly increases evaporation rates and thus heat loss. Fences or shrubs placed around the pool perimeter act as windbreaks (and increase privacy). Although painting the pool walls or bottom a dark color would increase solar heat gain, building codes will not allow it for safety reasons-you can't see anyone lying on the bottom.

Pools Get RSPEC from DOE

The Reduce Swimming Pool Energy Costs (RSPEC) program is an initiative of the U.S. Department of Energy's Institutional Conservation Program. RSPEC seeks to increase national awareness of pool energy consumption, related costs, and their impact on the environment.

As part of the program, DOE developed software, called Energy Smart Pools, so that pool managers, installers, and equipment suppliers can analyze pool energy conservation measures. The software analyzes both indoor and outdoor pools. Inputting information about the pool, including desired temperature, wind speed, fuel costs, and times of use, allows the software to estimate energy savings and payback from using solar pool heaters and pool covers.

The RSPEC software is available free of charge to energy auditors as well as professional pool design, installation, and management businesses and organizations. Contact Energy Efficiency and Renewable Energy Clearinghouse (EREC), P.O. Box 3048, Merrifield, VA 22116. Tel:(800)363-3732.

The pool itself is a solar collector. Shading during the day will increase the collector area needed to heat the pool. A pool cover will reduce the collector area required (see Putting a Cover on Heat Loss).

On average, the collector area must equal about 75% of the pool area to provide all of the pool heating. A key factor is how warm the users want the pool to be during the coolest month that they will use it. A higher temperature requires more collector area, which is more expensive.

In many cases, pool owners who already have a gas or electric heater will choose to use solar for only part of the pool heating load. This choice may be based on first-cost and space considerations. Collectors cost $150 to $200 each, and a site may have room to accommodate only three or four collectors.

Proper pump sizing is important for overall pool energy consumption and solar heater efficiency. It is possible to use an existing pool filter pump when installing a solar pool heater on an existing pool. However, this increases the amount of work the pump must do. The pressure in the filter is usually 10 to 15 pounds per square inch (psi). Depending on the type of solar pool collector, its distance from and above the pool, the diameter of the piping, and the number and type of elbows, the collector loop could add another 4-5 psi. If this pressure exceeds the filter capacity, a new pump, a booster pump, or a larger filter may be necessary. The diameter of the pipe to and from the collectors is typically 1 1/2 inches. Increasing the diameter to 2 inches, using 45o instead of 90o elbows, and using flexible hoses will reduce pressure losses in the system (see Pool Bills Take a Dive, EA&R Mar/Apr '86, p. 35).

Collector Siting and Mounting
Proper collector siting and mounting is important for maximum heating efficiency. The collectors should have complete exposure to the sun between 8 AM and 4 PM. Although true south orientation is best, orientation 15o east or west of true south will have little effect on the overall heating of the pool. Adding extra collector area can help to compensate for situations where the collectors cannot be sited within 15o of true south.

The collectors can be mounted on a roof, on a rack mounted on the ground near the pool, or on any smooth surface that has unobstructed solar exposure. Rubber mat collectors are usually glued to the roof.

The collectors also can be positioned at a fixed tilt to maximize their exposure to the sun during a particular season. The optimum tilt depends on when solar heat is needed most and on the site latitude. However, since setting the collectors at this tilt will typically increase performance by less than 10%, most systems are installed flat against sloped roofs. The expense of mounting collectors at a better tilt on flat roofs in northern latitudes (at least $100 for materials for a small application) has to be gauged against the benefits of improved collector performance.

Local building codes, covenants, and restrictions may affect how and where solar collectors can be sited.

System Costs and Economics
In most climates and situations, solar is a cost-effective replacement for electric heating systems. When installing pool heating equipment for the first time, solar is often more cost-effective than natural gas, propane, or electric systems. However, individual circumstances will vary.

Installing a conventional electric, propane, or natural gas heater for a 450 ft2 pool costs around $1,800. The cost of a solar pool heating system varies, depending upon location, seasonal use, and desired water temperature, which determine the amount of collector area needed. Solar collectors cost about $8-$12 per ft2 on average.

We used DOE's pool energy software to analyze typical pools in Michigan and Florida. We input a pool size of 450 ft2 and used average fuel costs for the two areas. We assumed use of a pool cover and that for 14 hours per week the pool was uncovered and used.

Installing a solar heating system instead of a conventional system on a new outdoor pool in Michigan came out very attractive economically. To maintain a minimum average temperature of 80oF from May 1 through September 30 in Detroit would require about 200 ft2 of solar panels. These would cost approximately $1,800 to install-about the same as a conventional system. Because a solar pool heating system has no fuel costs, it pays for itself the moment it is installed.

On the other hand, if the pool in Michigan is currently heated conventionally, retrofitting to solar is probably cost-effective only if the current system is electric. Propane could keep a 450 ft2 pool at 80oF from May 1 through September 30 for about $130 per season. Using natural gas would cost approximately $25 per season, while heating the pool with electricity would cost about $230 per season. The solar heating system's return on investment would be 8 years to replace the electric system, and 20 or 70 years to replace the propane or natural gas systems, respectively. However, this assumes the diligent use of the pool cover. A less effective cover, or a pool that is used all day long, could shift the results dramatically.

The economics of solar pool heating systems improve in Florida, where pools can be used all year. Maintaining a minimum temperature of 82oF in a 450 ft2 outdoor pool in Miami (again with an effective cover) for an entire year with propane, natural gas, and electricity costs about $280, $220, and $550 respectively.

For a new pool, a solar pool heating system in Florida came out cost-effective when compared to all conventional heating sources. Maintaining a year-round average pool temperature of 82oF in Miami in a 450 ft2 pool requires 360 ft2 of solar collectors. The installed cost is $3,240-almost twice that of a conventional heating system, but the payback from fuel savings is only 5 years compared to propane, 6.5 years compared to natural gas, and 2.5 years compared to electricity. Replacing existing conventional systems with a solar pool heater would pay back in 11, 15, and 6 years respectively.

Resources
The following organizations have information on solar pool heating: